An airplane is moved by several turbojet engines each housed in a nacelle also housing a set of attached actuating devices related to its operation and performing various functions when the turbojet engine is in use or stopped. These related actuating devices comprise in particular an electromechanical or hydromechanical system for actuating variable nozzle maneuvering. They can also comprise an electromechanical or hydromechanical system for actuating maneuvering of a thrust reverser system and a system for actuating cowlings designed to allow maintenance operations on the turbojet engine.
The role of the thrust reverser is, during landing of an airplane, to improve the braking capacity thereof by redirecting at least part of the thrust generated by the turbojet engine forward. In this phase, the reverser makes it possible to send all or part of the gas streams ejected by the turbojet toward the front of the nacelle, thereby generating a counter-thrust that is added to the braking of the airplane wheels. To do this, a thrust reverser comprises, on either side of the nacelle, a mobile cowling able to be moved between a deployed position, on one hand, that opens a passage in the nacelle intended for the streams deviated during a braking phase, and a retracted position, on the other hand, which closes said passage during the normal operation of the turbojet engine or when the airplane is stopped.
Currently, the actuating systems are primarily implemented via hydraulic or pneumatic cylinders. These cylinders require a transport grid for a pressurized fluid obtained either by air-tapping on the turbojet engine or by withdrawal on the hydraulic circuit of the airplane. However, such systems are bulky and require significant maintenance, because the slightest leak in the hydraulic or pneumatic grid can have harmful consequences both on the reverser and other parts of the nacelle. Moreover, the hydraulic or pneumatic cylinders also deliver the maximum possible power, which causes premature wear of the equipment.
To offset the drawbacks related to the pneumatic and hydraulic systems, nacelle builders and equipment manufacturers have sought to replace them and use electrical actuating systems as much as possible so as to lighten the nacelle and simplify its operation, in particular during the necessary maintenance cycles and the management of hydraulic or pneumatic fluids. Certain nacelle cowlings already exist designed for turbojet engine maintenance that are actuated by electric cylinders, and an electrically actuated thrust reverser is described in document EP 0 843 089.
Electric actuating systems allow optimal energy management as a function of the power actually necessary for the operation of these systems while also taking up less space in the nacelle and not requiring pressurized fluid circulation circuit. They also make it possible to integrate electronic control and steering systems, as described in French applications 04.07096, 07.07098 and 07.01058, for example.
Aeronautic regulations (FAR-JAR 25-933) require that thrust reverser control systems be protected from risks of untimely deployment through the establishment of a triple-locking system of the control members of the thrust reverser whereof the control must be segregated.
In the case of a hydraulic control system like the one that exists on the A340-500/600, each mobile reverser cowling has a tertiary bolt electrically steered by an independent device, and two so-called primary bolts installed in the upper and lower cylinders, the hydraulic control of which is allowed by joint steering of a first valve and a second valve for closing the hydraulic supply circuit of the cylinders. The steering of the two valves is done via two completely segregated steering lines.
Under flight conditions, the first valve remains closed and the hydraulic power is therefore not available to allow any unlocking of the primary bolts by the second valve alone.
In the case of an electrical system for actuating a thrust reverser, the movement command of the thrust reverser lever is first captured by a set of segregated computers.
A first computer is designed to control only the unlocking of the tertiary bolt, which therefore remains controlled by a dedicated control line.
The control of each primary bolt is authorized from a control unit receiving, on one hand, the necessary power supply controlled by a second computer and an opening order coming from a turbojet engine computer (FADEC or EEC).
Thus, when the pilot orders the opening of the thrust reverser, that order is captured by:                the first computer, which then orders the opening of the tertiary bolt,        the second computer, which then authorizes the power supply of the control systems of the primary bolts,        the turbojet engine computer, which, depending on the operating parameters of the turbojet engine that are representative of the flight phases, authorizes or denies the opening.        
One therefore understands that an electrical problem affecting the control of the tertiary bolt would not allow the unlocking of the primary bolts, since that control line is completely independent from it.
An electronic problem on the turbojet engine computer would also not by itself allow an untimely opening of the thrust reverser since, in the absence of an order from the second computer, no power supply is available.
Reciprocally, in case of error by the second computer allowing the power supply of the control systems of the primary bolts, the latter would not open since these control systems would not have the order from the engine computer.
The first and second computers generally use data from the aircraft not related to the engine, in particular, for example, altimetric data or data representative of the weight exerted on the wheels of the landing gear, inter alia.
The third computer, i.e. the engine computer, uses data representative of the operating system of the turbojet engine.
Such an architecture of the security system of the thrust reverser poses a problem when one wishes to group together, on a same power supply source, several functionalities used in flight, and more particularly a variable nozzle functionality. The interest of grouping several functionalities together on a same electrical power source is obvious. This avoids cluttering the nacelle and making it heavier with dedicated power supply systems for each functionality.
These two systems, thrust reverser and variable nozzle, have different operating moments, i.e. in landing phase and in coasting flight phase, respectively, during which the turbojet engine is in operation.
Thus, the electrical power delivered to the nacelle control system can no longer serve as discriminating security test since the power supply source can deliver electrical current during a usage phase of the airplane that does not concern the thrust reverser. The power supply therefore no longer serves as line of defense by itself.
More precisely, the test done by the second computer would be continuously validated, the output from the control units for the primary bolts would therefore be reduced to the order coming from the engine computer.
This third line of defense therefore needs to be restored so as to meet the air safety standards.